36 to 42 inches works for me, but there are other variables including height of table, use of foot stool or folding chair to stand on, and strength of table.
My benchwork is fairly strong, but I don't dare climb on top of it for fear that it might collapse if I put all my weight on it. I have local train buddies that do climb on their train tables because they used two by fours to build it.
In one place on my layout, it is 4 feet wide, so I left 9 to 10 inches between the back of the table and the wall so I can crawl under and rise up barely squeezing my upper body through that 9 to 10 inches of space to reach derailments, etc., so that is another option.
